1. Field
This invention relates generally to tube and shell steam generators and more particularly to cleaning apparatus for cleaning sludge from the secondary side from such a steam generator.
2. Description of Related Art
A pressurized water nuclear reactor steam generator typically comprises a vertically oriented shell, a plurality of U-shaped tubes disposed in the shell so as to form a tube bundle, a tube sheet for supporting the tubes at the ends opposite the U-like curvature, a divider plate that cooperates with the underside of the tube sheet and a channel head forming a primary fluid inlet header at one end of the tube bundle and the primary fluid outlet header at the other end of the tube bundle. A primary fluid inlet nozzle is in fluid communication with the primary fluid inlet header and a primary fluid outlet nozzle is in fluid communication with the primary fluid outlet header. The steam generator secondary side comprises a wrapper disposed between the tube bundle and the shell to form an annular chamber made up of the shell on the outside and the wrapper on the inside and a feedwater ring is disposed above the U-like curvature end of the tube bundle.
The primary fluid having been heated by circulation through the reactor enters the steam generator through the primary fluid inlet nozzle. From the primary fluid inlet nozzle, the primary fluid is conducted through the primary fluid inlet header, through the U-tube bundle, out the primary fluid outlet header and through the primary fluid outlet nozzle to the remainder of the reactor coolant system. At the same time, feedwater is introduced into the steam generator secondary side, i.e., the side of the steam generator interfacing with the outside of the tube bundle above the tube sheet, through a feedwater nozzle which is connected to the feedwater ring inside the steam generator. In one embodiment, upon entering the steam generator, the feedwater mixes with water returning from moisture separators supported above the tube bundle. This mixture, called the downcomer flow, is conducted down the annular chamber adjacent the shell until the tube sheet located below the bottom of the annular chamber causes the water to change direction passing in heat transfer relationship with the outside of the U-tubes and up through the inside of the wrapper. While the water is circulating in heat transfer relationship with the tube bundle, heat is transferred from the primary fluid in the tubes to water surrounding the tubes causing a portion of the water surrounding the tubes to be converted to steam. The steam then rises and is conducted through a number of moisture separators that separate entrained water from the steam and the steam vapor then exits the steam generator and is typically circulated through a turbine to generate electricity in a manner well known in the art.
Since the primary fluid contains radioactive materials and is isolated from the feedwater only by the U-tube walls, the U-tube walls form part of the primary boundary for isolating these radioactive materials. It is, therefore, important that the U-tubes be maintained defect free. It has been found that there are at least two causes of potential leaks in the U-tube walls. High caustic levels found in the vicinity of the cracks in tube specimens taken from operating steam generators and the similarity of these cracks to failures produced by caustic elements under controlled laboratory conditions, have identified high caustic levels as the possible cause of the intergranular corrosion, and thus possible cause of the tube cracking.
The other cause of tube leaks is thought to be tube thinning. Eddy current tests of the tubes have indicated that the thinning occurs on tubes near the tube sheet at levels corresponding to the levels of sludge that has accumulated on the tube sheet. During operation of a pressurized water reactor steam generator, sediment is introduced on the secondary side as the water changes to steam. This sediment accumulates as sludge on the tube sheet. The sludge is mainly iron oxide particles and copper compounds along with traces of other minerals that have settled out of the feedwater onto the tube sheet and into the annulus between the tube sheet and the tubes. The level of sludge accumulation may be inferred by eddy current testing with a low frequency signal that is sensitive to the magnetite in the sludge. The correlation between sludge levels and the tube wall thinning location strongly suggests that the sludge deposits provide a site for the concentration of a phosphate solution or other corrosive agents at the tube wall that results in tube thinning.
For the foregoing reasons, periodic cleaning of the sediment is desirable to maintain proper operation of the steam generator. Typically, spray nozzles are introduced along the center of the U-tubes (the tube lane) which move the sediment outward of the tube bundles. In the annulus, just outside the tube bundle, additional water flow is used to transport the sediment to a suction port where the sediment is carried outside the steam generator for disposal.
For some steam generators, such as those formerly manufactured by Combustion Engineering, Inc., the normal access for sludge lancing from the center of the steam generator outward is limited by restrictions in the tube lane. A divider plate located directly in the center of the tube lane restricts the horizontal access to a nominal 1 5/16 inch (2.85 cms.). Due to manufacturing tolerances, the space between the divider plate and the inner row of tubes can be closer to one inch (2.54 cms.). The additional space restriction is mostly due to the divider plate not being placed parallel to the inner row of tubes.
Since little space is available along the tube lane, presently cleaning is performed by sweeping high pressure and high volume water jets introduced along the periphery of the tube bundle of the steam generator. During cleaning, much of the spray is directed towards the center of the steam generator which pushes the sediment inward making it more difficult to remove. Another difficulty with spraying into the center of the steam generator is that the majority of the sludge deposits are further from the cleaning jets where the spray loses energy and focus. In addition, the jet spray is directed closer to being parallel to the tube sheet as opposed to being directed more perpendicular to the tube sheet where cleaning is more effective.
A challenge for effective sludge lancing is the ability to align the cleaning jets with the tube gaps, i.e., the space between the tubes. For Combustion Engineering designed steam generators, the gap between the tubes is nominally 0.116 inch (0.295 cms.). For deep penetration into the tubes, an angular alignment accuracy of +/−0.02 degrees is desirable. Gap and angular alignment are more difficult when spraying inward from the periphery as the jets must be repositioned with the tube gaps each time the fixture is moved.
Accordingly, it is an object of this invention to provide a sludge lance that can travel down the tube lane of a steam generator, between the divider plate and the first row of tubes without having its travel obstructed.
It is a further object of this invention to provide such a sludge lance that can readily be spaced a predetermined distance from the first row of tubes while being angularly aligned with the gap.
It is an additional object of this invention to provide such a sludge lance whose distance from the divider plate can be verified before set in operation.
It is an added object of this invention to provide such a sludge lance whose alignment does not have to be recalibrated after each movement.
It is a further object of this invention to provide support for a sludge lance nozzle that will counter any lateral reaction forces resulting from the high pressure fluid emanating from the nozzle jets.